1. Field of Invention
This invention relates to a system and method for improving the appearance of a digital image. More specifically, this invention is directed to a system and method for efficiently reducing image noise or "speckle" in digital images.
2. Description of Related Art
In the past, copiers or scan-to-print image processing systems were typically used to reproduce an input image as accurately as possible, i.e., to render a copy. Copies are intentionally rendered as accurately as possible, including any flaws present in the original input image. However, as customers become more knowledgeable in their document reproduction requirements, they recognize that an exact copy is often not actually desired. Instead, they desire to obtain the best document output.
Until recently, output image quality of a copier or a scan-to-print system was directly related to the input document quality. One common set of input documents includes photographs. Unfortunately, photography is an inexact science, and original photographs are often of poor quality. In addition, technology, age and/or image degradation variations often result in pictures having an unsatisfactory and undesirable appearance. Therefore, a copy giving the best possible picture is desired, rather than an exact copy of the original image.
Photography has long dealt with this issue. Analog filters and illumination variations can improve the appearance of pictures in analog photographic reproduction processes. For example, yellow filters enhance the appearance of white clouds against a blue sky in black and white images. Furthermore, various electrophotographic devices, including digital copiers, can clean up and improve images by adjusting threshold values, image filters and/or background suppression levels. Generally, these methods are manual methods in which a user must select various image adjustment operations on an image by image basis. Typically, the casual user is not skilled enough to perform these operations, especially when the electrophotographic device includes color controls.
Three possible choices are currently available to enhance an image. The first choice is doing nothing. Such a system is a stable system, in that it does no harm to an image. This is a common reproduction approach. However, the output quality of documents produced by these systems are sometimes not satisfactory to the ultimate customer.
The second choice is to process all images. An improvement can usually be made to an image if certain assumptions are made that are accurate for most cases. In an exceptionally large subset of images, increasing contrast, sharpness and/or color saturation will improve these images. This reproduction process tends to produce better images. However, the process is unstable because, for multi-generation copying, increases in contrast, saturation or sharpness are undesirable and ultimately lead to severe image degradation. Furthermore, the process may undesirably operate on high quality images.
The third choice is an automated image enhancement process which operates to vary images which are not perceived as high quality images, but does not operate on images which do not need to be improved.
Many improvements can be made to an image using automated image enhancement techniques, including exposure adjustment, described in U.S. Pat. No. 5,414,538 to Eschbach, color balance correction, described in U.S. Pat. No. 5,371,615 to Eschbach, and contrast enhancement, described in U.S. Pat. No. 5,450,502 to Eschbach et al., each herein incorporated by reference in their entireties.
Generally, these processing methods operate by modifying a set of tone reproduction curves. The output image is achieved by using tone reproduction curves, operating either on the luminescence channel expressed in LC.sub.1 C.sub.2 coordinates or, preferably, on each channel in a color density space description of the image in red-green-blue (RGB) coordinates. A method of cascading or serially ordering such processing is described in U.S. Pat. No. 5,347,374 to Fuss et al., herein incorporated by reference in its entirety.
The automated image enhancement techniques described above attempt to automatically improve the perceived quality of natural scene images by estimating and appropriately modifying the overall exposure, contrast, color balance, saturation and sharpness of the input image. However, one source of image degradation, image noise or "speckle", is not addressed by these techniques. Image noise is not addressed by the above automated image advancement techniques because a noise filter, including even a simple low-pass filter, takes a considerable amount of time to operate on the input image. This undesirably reduces the overall performance of the system. Specifically, a noise filter reduces performance of an automated image enhancement system by as much as 50%.